Thermal activation in spin-polarized current driven magnetization reversal process

摘要

Magnetization reversal by spin polarized current flowing perpendicular to a 10 nm Co/ 6 nm Cu/ 2.5 nm Co circular nanopillar of 130 nm diameter is studied using a micromagnetic model. In addition to the spin transfer torque term, the magnetostatic coupling with the pinned layer and the classical Ampere field created by the current are taken into account. Furthermore, thermal activation is also considered by means of a stochastic white-noise thermal field. The experimental parameters are J_0 = 4.5 x 10~8 A/cm~2, B_(ext) = 0.14 T, T=300 K, whereas the damping parameter is α = 0.005. Firstly computations without thermal activation were performed. Transitions from the parallel state (PS) to the antiparallel one (APS) and backwards were computed. In both cases, magnetization reversal occurs via highly inhomogeneous states revealing that these processes are not accurately described by single-domain models. Considering the non-equilibrium character of the switching process, the thermal activation simulations have to be carried out as many times as possible in order to average for obtaining quantitative results. Thermal activation favors the transition from PS to APS, a decrease in the switching time is observed and the intermediate state which appears in the computations without thermal field is suppressed. On the contrary the transition from APS to PS is not so influenced and an intermediate magnetization state remains stable even after averaging over 100 realizations.